HVDC (High Voltage Direct Current) is a well-proven technology for transmitting electric power over long distances, and provides an alternative to the more conventional AC transmission. Most HVDC power transmission systems comprise point-to-point connections, but HVDC power transmission systems comprising multi-terminal connections, especially for voltage source based HVDC converters, are experiencing growing interest and are under intense development.
In the HVDC power transmission system it is most important to be able to establish communication between the terminals, i.e. between the converter stations. This is particularly true in the multi-terminal system as such systems entail somewhat more complex control schemes compared to point-to-point systems. In the multi-terminal system, aspects such as for example balancing of power orders between the different converter stations, managing of overload and relocating power between converter stations are more complex as a particular converter station may be interconnected to more than one other converter station.
The communication between the converter stations thus comprises for example exchange of control data such as power scheduling orders and power levels. The communication means conventionally comprises an external, separate data communication system and/or telecommunication network to which all converter stations are connected and by means of which the control data may be exchanged.
The HVDC power transmission system has to be able to operate even in case of failure of the data communication system and/or telecommunication network and is therefore designed for such cases. However, loss of the external communication means would lead to a situation in which the converter stations have to rely on backup schemes e.g. utilizing estimations, non-updated control data and/or control data based only on data available at the respective converter station. To operate the HVDC power transmission system based only on local information is obviously less than optimal and the exchange of control data between the converter stations is therefore much desired for obtaining optimal operation of the HVDC power transmission system.
In “A novel control technique to operate DC/DC converters in parallel with no control interconnections”, by A. Tuladhar and H. Jin, a control method is described for load sharing. A small ac voltage is injected into the system and used as a control signal. However, such method would require costly equipment on the high voltage side for modulating a high frequency carrier signal onto the transmission line. Further, such method is not practical in a large DC grid with high line capacitance to ground which will require high injection power levels.
From the above it is clear that it would be desirable to be able to alleviate the need for and dependence on external communication means such as the separate data communication system and/or telecommunication system.